1. Field of the Invention
The present invention relates to a machine tool such as, for example, a lathe, a machining center, a grinding machine, or a drilling machine and, more specifically, to correction of a thermal displacement occurring therein.
2. Description of the Related Art
In a machine tool such as, for example, a lathe, a closed loop control, which includes a feedback control system, has been utilized in controlling feeding a tool post or a head stock employed in a lathe of a spindle moving type. While a semi-closed loop system is generally utilized which makes use of a pulse coder that is accessory to a servo motor, a full closed loop system, which controls the position of, for example, a tool post by reading the position of the tool post by means of a linear position detecting unit such as, for example, a linear encoder, is often used where a highly accurate positioning is required.
Also, the machine tool is susceptible to the thermal expansion and/or the thermal deformation occurring in a bed or any other site because of cutting heat and/or heat emission at various sites thereof as a result of the machine operation. The thermal expansion and/or the thermal deformation lead to a reduction in processing accuracy. Accordingly, some of the machine tools make use of a cooling device to minimize the thermal expansion and deformation. However, where the thermal expansion is desired to be sufficiently suppressed, the cooling device tends to become bulky or mere cooling is unable to secure a sufficient processing accuracy.
In view of the foregoing inconveniences, various suggestions have been made to measure the temperature to enable the thermal displacement occurring in the machine as a whole to be estimated so that a thermal displacement correction such as, for example, a correction of the cutting quantity (cutting depth) of a cutting tool can be implemented. It has however been found that where the displacement is estimated from the temperature detected, a complicated calculation such as, for example, a time delay is required at the stage at which the displacement is calculated from the temperature. Considering that change in temperature originates from various factors, it is indeed difficult to implement the thermal displacement correction precisely from the measurement of the temperature. Because of it, measurement of the actually occurring thermal displacement with the use of, for example, a linear scale has been suggested to perform the correction.
By way of example, the machine tool disclosed in, for example, the JP Laid-open Patent Publication No. 2002-144191 is a lathe which is so configured and so structured that, as shown in FIG. 8 of the accompanying drawings, a head stock 51 is fixedly positioned on a bed 52 and a feed slide 54 having a tool post 53 mounted thereon is movable in a direction radially of a spindle (X-axis direction). In this prior art lathe, a scale 55 fitted to the head stock 51 so as to extend in the direction radially of the spindle is read by a reading member 56, fitted to a feed slide 54, to thereby measure the position of the tool post 53 in the direction radially of the spindle. The measurement of the position of the tool post 53 in the direction radially of the spindle changes with, for example, the thermal deformation. Accordingly, in this prior art lathe, the proper processing accuracy is secured at all times by correcting, for example, the cutting quantity of a tool 57 according to the measurement.
Measurement of only the relative position between the head stock 51 and the feed slide 54 such as disclosed in JP Laid-open Patent Publication No. 2002-144191 referred to above results in an error in the distance between a spindle axis and the tool in the event of occurrence of the thermal displacement of the tool post 53 relative to the feed slide 54.
According to the conventional control of the full closed loop system, for example, the lathe of a tool post moving type is controlled by directly reading the movement of the tool post by means of the linear position measuring instrument such as, for example, a linear encoder. It has however been found that, in the event of the thermal displacement occurring at a site at which the linear position measuring instrument is fitted, the control is no longer accomplished with a high accuracy. As a result, the conventional control of the full closed loop system requires the lathe to be equipped with extra instruments for performing a thermal displacement correction.
Also, since the processing dimension of a workpiece is determined by the distance between a workpiece center and a blade tip of the tool, a direct detection of this distance if possible makes it possible to accomplish the processing with a high accuracy. It has however been found that the direct measurement of the distance between the workpiece center and the blade tip of the tool requires a measuring instrument to interfere with the workpiece and/or the tool and, therefore, the direct measurement of such distance has not yet been realized.